Abstract The field of medical x-ray imaging experienced a digital revolution in the early 2000s, with the spreading of digital radiography (DR) systems which are based on active matrix flat panel imagers (AMFPI). Since then we have seen rapid development and clinical translation of large-area AMFPI based on amorphous silicon (a-Si) active matrix technology. Because of their compact size, rapid image readout and excellent image quality, AMFPI is being used not only for traditional x-ray imaging modalities such as general radiography and fluoroscopy, but also in tomographic imaging applications including cone beam computed tomography (CBCT) and digital tomosynthesis. It is believed that AMFPI will become the dominant detector technology for x-ray imaging. However before AMFPI can equal the performance of the x-ray image intensifier (XRII) in fluoroscopy, they have one major difficulty to overcome: producing x-ray quantum noise limited images at very low dose, such as in the dark part of a fluoroscopy image (0.1 R per frame). Our hypothesis is that an indirect-conversion x-ray flat-panel imager with avalanche gain and cadmium selenide (CdSe) photoelectric conversion layer will provide an enormous dynamic range and efficiency that permits x-ray quantum noise limited performance from a single x-ray photon level in low-dose fluoroscopy all the way to high dose radiography. The objective of this proposal is therefore to fabricate and test a prototype high-efficiency flat-panel x-ray imaging detector with avalanche gain. Conceptually the proposed detector employs four major components: (1) a structured cesium iodide (CsI) scintillator to convert x-rays to optical photons; (2) an ultra-high efficiency CdSe photoelectric conversion layer to convert the optical image to charge; (3) field shaping avalanche amorphous selenium (a-Se) photoconductor, called field shaping HARP (High-gain Avalanche Rushing amorphous Photoconductor), to amplify charge via avalanche multiplication gain; and (4) a large area active matrix (AM) thin film transistor (TFT) array to read out the image electronically in real-time. The proposed detector is known as field shaping SHARP-AMFPI (Scintillator-HARP Active Matrix Flat-Panel Imager). Successful development of field-shaping SHARP-AMFPI will lead to the clinical-translation ready, practical low-dose x-ray imaging technology, which will be the first ever large area solid-state detector with stable avalanche gain.